The present invention relates to a valve system for controlling the intake pressure of a fluid in a high-pressure pumpxe2x80x94e.g. a fuel for supply to an injection enginexe2x80x94and to a relative overpressure valve.
As is known, when a variable quantity of high-pressure fluid is required, the maximum quantity of fluid is normally compressed, and the delivery pressure of the pump is controlled by a first overpressure valve which drains off the surplus high-pressure fluid. The intake pressure of the fluid is in turn controlled by a second overpressure valve which drains off the surplus low-pressure fluid.
In the case of fuel supply to an injection engine, the high-pressure fuel is supplied by a high-pressure, normally piston, pump in turn supplied from the fuel tank by a low-pressure pump.
Known supply devices require two separate pressure control valves: one for controlling the high pressure of the fuel downstream from the high-pressure pump, and the other for controlling the pressure of the fuel entering the pump. Valve systems of this sort are therefore complicated and expensive.
Moreover, energy is obviously wasted by the overpressure valve downstream from the high-pressure pump recirculating back into the tank the surplus fuel pumped by the high-pressure pump. And since compression generates heat, this enters the fuel in the tank, thus resulting in an increase in the temperature of the fuel to be pumped. This in turn increases fuel leakage of the pump pistons, thus reducing the efficiency of the pump, so that a cooler may also be required.
It is an object of the present invention to provide a valve system for controlling the fluid intake pressure of a pump, and which provides for maximum efficiency, is low-cost, and eliminates the aforementioned drawbacks typically associated with known valve systems.
According to the present invention, there is provided a valve system for controlling the intake pressure of a fluid in a high-pressure pump, comprising an on-off valve for the fluid entering said pump; characterized in that the intake pressure of said pump is controlled by an overpressure valve communicating with said on-off valve and for draining any surplus fluid for supply to said pump.
More specifically, the fluid is a fuel for supply to an injection engine, the on-off valve is a variable-capacity electromagnetic valve, and the overpressure valve communicates with the delivery side of a low-pressure pump.
Preferably, the overpressure valve comprises a valve body having a cylindrical cavity, in which slides a cylindrical shutter comprising a lateral wall and an end wall; a calibrated spring being located outside the shutter, between the end wall and a member fitted inside said cavity.